"The truth is, I think there's a little bit of sour grapes going on here because he's not really included much in the discussion about how they are going to go about this negotiation, partly because... I think ministers don't fully trust him," the ex-Work and Pensions Secretary told Sky News.

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“What surprises me is the deafening silence from ministers who should be taking to the airwaves to defend the integrity and capability of the impartial civil service.

"The Prime Minister herself has publicly criticised civil servants, trivialised those who suggest that the civil service is being under-resourced and now sits back as key officials are pilloried by a succession of former ministers.

"If the civil service is to deliver a successful Brexit negotiation, the recipe for that success is unlikely to be to starve it of resources, lack clarity of objective and be surrounded with yes men and women who will not speak truth unto power."

European Commission spokeswoman Natasha Bertaud said: "We regret the loss of a very professional, very knowledgeable - while not always easy - interlocutor and diplomat who always loyally defended the interests of his Government."

And the European Parliament's chief Brexit negotiator, Guy Verhofstadt, paid tribute by describing him as "a much-respected UK civil servant in Brussels who knew what he was talking about".

But Tory former trade secretary Peter Lilley accused Sir Ivan of "sour grapes", as he called for the ambassador to be replaced with someone tough enough to walk away from negotiations with no trade deal at all.

The ex-Cabinet member told BBC Radio Four's World at One: "Why did he put out this long, 1,400-word letter, half of which reads like a sort of CV - this is my job, I'm a very clever person, anyone who is looking for someone to employ, I'm available.

"And the other half looks like some degree of sour grapes, and possibly some genuine degree of misunderstanding between him and ministers, or concern about what ministers are doing.

"It would be very unusual to have somebody leading the team who was not really committed to leaving. And one has a feeling that many diplomats, Eurocrats, are actually in the business of trying to negotiate a way back in, rather than committing to getting us out.”

But former Foreign Office permanent secretary Lord Ricketts attacked the "denigration" of Sir Ivan on the same programme.